Grounding plug

ABSTRACT

An electrical connector plug for providing connection to two or more sides of a circuit with conventional prong means and having a grounding prong which can be inserted into a grounding socket in a receptacle. The grounding prong is provided with a protruding sensing finger that is in an inactive position when the receptacle has a grounding socket, but if there is no grounding socket present, then the protruding sensing finger strikes a solid surface of the receptacle and moves backward to trigger a latch means that unlatches the grounding prong and allows it to be pushed back into the body of the plug, allowing the circuit contact prongs to enter the receptacle&#39;&#39;s live sockets to complete the electrical circuit. In order to prevent accidental triggering of this sensing finger by finger pressure on the end of the sensing finger or by force exerted on the axially-shiftable portion of the grounding prong, which might overcome the safety feature of the grounding prong, by finger pressure on the end of the grounding prong, the mechanism may be safetied by an arrangement for the latch means, such that pressure applied to the protruding prong by a yieldable means such as a finger, or other yieldable portion of the body, will push the entire grounding prong back toward the plug but will prevent the protruding sensor finger from triggering the lever, therefore preventing the retraction of the grounding prong into the plug. As an additional feature, the grounding prong can be designed so that it will never be completely retracted into the plug and a grounding plate is provided for attachment to a receptacle so that when the grounding prong is used in conjunction with a receptacle not having a matching grounding socket, the retracted grounding prong will still contact the grounding plate and provide for the safe grounding of the circuit.

United States Patent [191 Nelson [451 Aug. 21, 1973 1 1 GROUNDING PLUGRichard B. Nelson, 3160 Hollycrest Dr., Hollywood, Calif. 90028 [22]Filed: Sept. 30, 1971 [21] Appl. No.: 185,123

[76] Inventor:

[52] 11.8. C1. 339/14 P, 339/31 R [51] Int. Cl H0lr 3/06 [58] Field ofSearch 339/14 R, 14 P, 14 RP,

[56] References Cited UNITED STATES PATENTS 3,308,415 3/1967 Cramer eta1. 339/14 R 2,984,808 5/1961 Bender 339/14 RP 2,299,390 1/1967Eckelkamp..... 339/12 3,025,486 3/1962 Falconer 339/31 R 3,440,5914/1969 Whalen 339/14 R Primary Examiner-Marvin A. Champion AssistantExaminerRobert A. l-laffer [57] ABSTRACT cle has a grounding socket, butif there is no grounding socket present, then the protruding sensingfinger strikes a solid surface of the receptacle and moves backward totrigger a latch means that unlatches the grounding prong and allows itto be pushed back into the body of the plug, allowing the circuitcontact prongs to enter the receptacles live sockets to complete theelectrical circuit. In order to prevent accidental triggering of thissensing finger by finger pressure on the end of the sensing finger or byforce exerted on the axially-shiftable portion of the grounding prong,which might overcome the safety feature of the grounding prong, byfinger pressure on the end of the grounding prong, the mechanism may besafetied by an arrangement for the latch means, such that pressureapplied to the protruding prong by a yieldable means such as a tinger,or other yieldable portion of the body, will push the entire groundingprong back toward the plug but will-prevent the protruding sensor fingerfrom triggering the lever, therefore preventing the retraction of thegrounding prong into the plug. As an additional feature, the groundingprong can be designed so that it will never be completely retracted intothe plug and a grounding plate is provided for attachment to areceptacle so that when the grounding prong is used in conjunction witha receptacle not having a matching grounding socket, the retractedgrounding prong will still contact the grounding plate and provide forthe safe grounding of the circuit.

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PAIENIEDmcznm 3754;202- sum 2 or 2 i INVENTOR. RICHARD 5. NELSON vb.6-2a- ATTOR/VA'YS.

1 GROUNDING PLUG BACKGROUND OF THE INVENTION This invention relates toan electrical plug for attachment of a source of electrical powerthrough an electrical cord to an electrical circuit or machine, withmeans provided for a separate grounding contact.

Household electrical appliances, portable machinery, and other devicesutilizing electrical circuitry, which are connected to a power sourcethrough electrical cords having a plug installed on the end with whichto engage an electrical receptacle have conventionally had two or moreconductors in the electrical cord connected to prongs in a plug, whichengage contacts in sockets in the electrical receptacle. In order toreduce the hazards of electrical shock to the user of the appliance, aseparate grounding wire has been used in the electrical cord, thegrounding wire being attached to the frame of the appliance and to agrounding prong on the electrical plug, which will be engaged with agrounding socket in the electrical receptacle. In some other varietiesof plugs, the grounding wire has protruded from one side of the plug,and was adapted to be attached to a grounded screw on the receptacleafter the plug had been inserted into the normal power sockets in theelectrical receptacle. However, attaching the ground wire after pluggingthe plug into the receptacle is dangerous if a short circuit existswhich would energize the frame of the appliance, and it was ofteninconvenient to attach the wire before the plug was inserted into thereceptacle.

Further, an appliance equipped with a grounding plug having a protrudinggrounding prong could not be engaged into a receptacle that did not haveprovision for receiving the grounding prong into a grounding opening forcompleting the grounding function. Therefore, adaptors have to beprovided and kept on hand in such cases. These adaptors are sometimessupplied as three pronged grounding plugs, but often they becomeseparated and lost causing the user to accomplish the insertion of theplugs by breaking off the grounding prongs of the connector, or by otherhazardous means.

Prior art such as US. Pat. No. 3,171,113 dated Feb. 23, 1965 for AnElectric Plug with Ground Indicating Light" provides for manualretraction of a grounding prong for use in a socket not adapted forgrounding. However, such a device would leave the grounding prongretracted until such time as it is manually released. Thus, the usercould use the plug in a three prong grounded socket with the groundingprong held in a retracted position, thus defeating the purposes of anautomatic safety grounding device.

Similar disadvantages may be seen in the Three Way Electric Plug,patented Mar. 13, 1962 under U.S. Pat. No. 3,025,486 which achieves themanual retraction of a grounding prong by means of a push button whichretracts and locks the grounding pin in a retracted position and thuscan defeat the safety purpose of the grounding prong.

U.S. Pat. No. 2,323,736 issued on July 6, 1943 for an Attachment Plug,"has similar propensities for overcoming the safety features of thegrounding device as do US. Pat. No. 3,308,415 issued on Mar. 7, 1967 fora Self-Adjusting Three Pronged Electric Plug Adapted for Either Two orThree Pronged Electric Sockets," and US. Pat. No. 2,922,134 for anElectrical Connector Plug issued on Jan. 19, 1960.

In contrast to the above reference background patents, an object of thisinvention is to provide a grounding electrical attachment plug which canbe used with socket receptacles having two or more receiving apertures,but providing for automatic retraction of the grounding prong when acorresponding receptacle aperture is not present.

A further object of the invention is to provide for spring biasing ofthe grounding prong to hold the prong in an extended position unless itis forcibly retracted and held in a retracted position by the mechanicalurging of a contact with a receptacle surface. A further object of theinvention is to provide for a mechanical latch means which will hold anextended grounding prong in position in a manner such that pressure ofinsertion of the grounding prong into the receptacle will not cause itto retract. A further object of the invention is to provide a sensingfinger protruding from the grounding prong to release the latch means ifno matching aperture exists in the electrical receptacle.

A further object of the invention is to provide a safety device suchthat pressure applied to the sensing finger of the grounding prong byresilient means such as a human tissue, will simply move the groundingprong backwards together with the sensing finger and prevent release ofthe grounding prong for telescoping motion into the plug body.

Still another object of the invention is to provide an electricgrounding means for a receptacle that is not provided with a groundingaperture. Yet another object is to provide for automatic contact of thegrounding prong with an electrical grounding means provided externallyon an electrical receptacle.

All of these objects and advantages of the present invention aredesigned to provide a grounding plug which cannot have its safetyfeatures easily overcome by the user and which will provide automaticgrounding of plugs in either grounded receptacles having grounding prongreceiving means, or in grounded receptacles to which external groundingmeans can be added for automatic contact with partially retractedgrounding prongs. In all cases the grounding prong retraction isindependent of the user of the grounding plug and is accomplishedautomatically when the prongs of the connector plug are inserted into anelectrical receptacle.

The above and other features of this invention will be fully understoodfrom the following detailed description and the accompanying drawings,in which;

FIG. 1 is an axial view in compound cross-section of the presentlypreferred embodiment according to the invention;

FIG. 2 is a right hand end view of FIG. 1;

FIG. 3 is a cross-section taken on line 3-3 of FIG.

FIG. 4 is a cross-section taken on line 4-4 of FIG.

FIG. 5 is a fragmentary cross-section of the embodiment of FIG. 1 withparts shown in displaced position;

FIG. 6 is a fragmentary cross-section of another embodiment of theinvention;

FIG. 7 is a cross-sectional view taken on line 7-7 of FIG. 6;

FIG. 8 is a side view of the embodiment of FIG. 1 shown in partialcross-section, and in assembly with an electrical receptacle;

FIG. 9 is a view taken on line 9-9 of FIG. 8.

FIG. 10 is a section taken at line 10-10 of FIG. 9;

FIG. 11 is a bottom view of a part of the assembly of FIG. 9;

FIGS. 12 and 13 are plan views of alternate constructions for a part ofthe assembly in FIG. 9',

FIG. 14 is an end view of another connector useful with the invention;

FIG. 15 is a side view of another receptacle useful with the invention;

FIG. 16 is a side view of a connector complementary to the device ofFIG. 15;

FIG. 17 is an end view taken at line 17-17 of FIG. 15; and

FIGS. 18 and 19 are side and top views, respectively, of an alternateembodiment for the device of FIG. 15.

The preferred embodiment of this invention is shown as connector plug14. This plug includes a body 15, made of insulative material, with anentry port 16, which is adapted to receive an electrical cord 17. Theelectrical cord 17 contains three conductors 18, 19 and 20 in the formof insulated wire. The body 15 has two electrically conductive prongmembers 21 and 22 embedded within the body 15 and extending therefrom.The body 15 is an elongated structure having a longitudinal axis 23 andthe prong members 21 and 22 are held parallel to each other and to axis23. Electrical conductors 18 and 19 are secured to the prong members 21and 22 for electrical contact and transmission of electrical currentflow. Conductor 20 is attached to v a grounding prong 24a.

Grounding prong member 24a extends parallel to axis 23 of the plug 14,and includes an axially shiftable conductive element 24 and an outertube 28. Outer tube 28 may be conveniently molded or otherwise fixed inposition when the body 15 is initially manufactured. It is oftenconvenient to make a body such as body 15 in a plastic molding process,in which case outer tube 28 can be molded in position by preplacementwithin a die cavity. Intermediate tube 29 is inserted into the outertube 28. The outer tube 28 is then swaged to fold over the protrudingend 30 of outer tube 28, and the swedged end prevents the intermediatetube 29 from being withdrawn from the assembly. Outer tube 28 has oneend 31 flattened and modified for attachment of the conductor 20. Thecombination of the outer tube 28, intermediate tube 29, and axiallyshiftable conductive element 24 form a grounding prong member. Thecombination of the outer tube 28 and the intermediate tube 29 issometimes referred to as an outer tubuler member which is fixed to thebody. Intermediate tube 29 is provided with a rectangular slot 32piercing one wall from the exterior surface to an axial passage 33. Across pin 34 (FIG. 4) is installed transverse to the axis of the outertube 28 and the intermediate tube 29 to hold them in a relative fixedposition so that the intermediate tube 29 which is already restrainedfrom being withdrawn from the outer tube 28 by the swedged end 30 willbe prevented from being pushed further into the body 15. A compressionspring 35 is contained within the passage 33 of intermediate tube 29 andis reacted against the transverse pin 34. The axially shiftableconductive element 24 is slidably inserted into the bore 33 of theintermediate tube 29 that is, it is shiftable in, and embraced by, thewall of passage 33.

Axially shiftable conductive element 24, which is of hollow tubularconstruction, has a transverse bellcrank lever pin 36 (FIGS. 1 and 3)mounted to it on a diameter transverse to the longitudinal axis of thegrounding prong 24a. Pin 36 extends at either or both of its ends intoone or both of slots 36a and 36b in intermediate tube 29. This holds theaxially shiftable conductive element 24 against rotation and againstexpulsion from the body. Swivally mounted to the pin 36 is a bell-cranklever 37 which has two protruding lever arms 38 and 39, which aredisplaced at right angles to each other. The lever 37 and its arm 39 aresometimes called latch means." Slot 32 in the intermediate tube 29 isaxially aligned with slot 40 of axially shiftable conductive element 24.Bias spring 35 presses against lever arm 39 (sometimes called anupstanding" lever arm) to bias it toward its extended position sometimescalled an upstanding position as illustrated in FIG. 1) wherein itstands in slots 32 and 40 and exerts a latching action yet to bedescribed. A cap plug 42 (sometimes called cap plug means) is installedin the clyindrical bore 43 of the axially shiftable conductive element24. Capplug 42 has a longitudinal bore 44 which slidably mounts asensing pin 45. Cap plug 42 is constructed of electrically conductivematerial. Sensing pin 45 protrudes beyond the face 46 of cap plug 42 butis prevented from withdrawal from cap plug 42 by a formed protuberance47 intermediate on the shank of sensing finger 45 (sometimes calledsensing finger means"). Sensing finger 45 is restrained at its internalend by lever arm 39 of a bell-crank lever 37.

For convenience of construction, and of making electrical connections,the body 15 of the electrical plug may be made in two sections 50 and 51and these sections may conventionally be held together with screws suchas 53, or any other convenient clamping method. The cap plug 42 shouldmake a tight fit in the grounding prong 24 to prevent accidentalexpulsion.

In use in a conventional grounding receptacle where there would besocket apertures to receive the electrical contact prongs 21 and 22, andthe grounding prong 24a, the plug 14 would act as a conventional threeconductor grounding plug. However if there is no provision for thegrounding prong member 240 in the receptacle then the end of sensingfinger 45 would be pushed back to the solid face of the receptacle as isshown in FIG. 5. Pushing back sensing finger 45 removes the lever arm 39of the bell-crank lever 37 from the slot 32 of intermediate tubularmember 29, thus permitting axially shiftable conductive element 24 tobepushed backwards against compression spring 35 into the bore 33 of theintermediate tubular member 29. It will be observed that bias spring 35yieldingly biases element 24 toward the protruding position of FIG. 1,and that it also yieldingly rotatably biases bell-crank lever 37 to wardits latched (upstanding) position shown in FIG. 1. However, if anindividual tried to defeat the protection of the grounding prong 24 byapplying finger pressure (the finger being yieldable tissue) to thesensing finger 45, the entire element 24 with the sensing pin 45 wouldtravel backwards for the length of slot 32 in intermediate tubularmember 29 without causing sufficient retraction of the sensing pin toturn lever 37 sufficiently to remove it from slot 32 before the fingeralso contacts the end of element 24. Then element 24 moves back alongwith the sensing finger. As soon as lever arm 39 contacted the end ofslot 32, it would be impossible for the sensing finger 45 to release thelever arm 39 from engagement with slot 32 and therefore the axiallyshiftable conductive element 24 would remain locked in relative positionto the intermediate tubular member 29 and could not be retracted intothe bore 33. This prevents manual override of the safety system. Thissame result would occur were retraction attempted by pushing on element24 without first retracting the sensing finger. This prevents retractionby frictional engagement with the wall of the receptacle as the element24 is pressed into it. When the plug 14 is withdrawn from the receptaclea retracted element 24 will automatically spring forward to protrudefrom the body and be locked in its grounding position by bell-cranklever 37.

As an ancillary improvement, a butterfly shaped grounding plate 55 maybe attached by means of the center plate mounting screw 56 which isconventionally used in wiring receptacle boxes, and the grounding plate55 is made of the length that will contact the faces of receptaclesockets 57 and 58 (FIG. 13). Since a receptacle box 59 is normally agrounded portion of the circuit, the ground will be transmitted throughthe screw 56 and through the conductive member 55. Therefore, when theplug 14 is inserted into the receptacle 57 the axially shiftableconductive element 24 is pressed into the bore 33 against thecompressive spring 35 bias, which in turn presses the element 24 intocontact with the grounding plate member 55, therefore providing anautomatic ground. There can be no defeat for this system because theelement 24 is spring loaded to protrude for whatever distance is allowedby the receptacle.

Grounding plate 55 is preferably made so it can be snapped into place.Accordingly, its central region 70 has a hole 71 therethrough to passthe grounding screw, and also four fingers 72 bent away from the centralregion to clamp onto the edges which bound apertures 73, 74 in a coverplate. Contact surfaces 75, 76 project to the area where they will becontacted by the element 24. The surfaces are aligned by the contactmade between the cover plate and the fingers. Plate 55 can thereby besnapped into place before being held by the screw.

Depending on the orientation of the apertures in the receptacle whichare exposed through apertures 73 and 74, it is possible that there maybe a situation where a contact surface may not be contacted by theelement 24. Accordingly, an extension 80 (FIG. 12) may be provided atone side of the central region, or two extensions 81, 82, one at eachside (FIG. 13). The arrangements of FIGS. 12 and 13 can assure thatthere is no receptacle arrangement wherein a grounding action does notoccur.

FIG. 14 shows a plug which is coded so there will be no possiblereversal of polarity, and grounding would always be assured. In thisconstruction, a plug 85 includes a grounding prong member 86 accordingto this invention, plus a larger and a smaller conductive prong 87, 88,respectively whose sizes are coded to the hole size in the receptacle soprongs 87 and 88 cannot be reversed. i

FIGS. l5, l6 and 17 show a reversal of parts such as might occur where acord set is used with an appliance. In such an arrangement, an appliance90 such as a coffee pot, is provided with two circuit prongs 91, 92 anda retractable grounding prong member of the type shown in FIG. 1. Thereare mounted to the appliance rather than to a cord-carried plug.

A cord-carried receptacle 94 is attached to a threeconductor cord, andincludes apertures 95, 96 for prongs 91 and 92, and an aperture 97 formember 93. When aperture 97 is present, the prong enters it. If it isabsent, prong 93 retracts.

Incidentally, an extra ground lead 100 is shown attached to the groundlead. It has a spade attachment 101 to be attached to a grounded screwif desired. If this feature is not used, lead 100 is merely cut off.

FIGS. 18 and 19 illustrate another possible usage of the device of FIG.16. It may be that, rather than insert the ground prong into a groundedaperture, it would be preferred for a springy abutment contact to bemade. Accordingly, a grounded strap 102 may be bent over the end ofreceptacle 94, and when it is attached to the appliance, prongs 91 and92 enter the respective apertures, but member 93 will be triggered, andwill be pressed inwardly against its bias spring. This will constitute areliable ground for the appliance.

An alternate construction to accommodate the sensing finger 45 is shownin FIG. 6 where the sensing finger 45 lies adjacent to the element 24 ina channel formed in the exterior of the element 24, rather than totallywithin the element. What does exist in common is that the end of thesensing finger is spaced axially farther from the face of the plug thanis the end of element 24.

As an additional feature, cap plug 42 may be made of a magnetizedmaterial and the conductor 55 may be made of a magnetically permeablematerial so that the element 24 will be magnetically held in firm,mutual contact with the conductor member 55.

It is to be understood that the grounding conductor 20 would be groundedto the frame of an appliance or other electrical device, and that theground receptacle or contact within the receptacle to receive the plug14 would be connected to electrical ground.

In all embodiments, where the element 24 and the intermediate tube 29 ortheir equivalent are defined as tubular, the term tubular is notintended to be limited to a structure having concentric walls, or even acylindrical wall. It is intended to define elements of an assembly whichwill telescope, and which will receive such internal elements as arerequired.

The shape of the outside wall of the element 24 can be selected to fitapertures of any shape, all of which fall within the scope of thisinvention. 1

Therefore, this electrically grounded plug provided the convenience of amultiple pronged plug system having an automatically retractablegrounding prong member that is actuated only when a receptacle is notprovided for such a member, but yet the prong can be safe from manualtampering by the user since it cannot be actuated except by the surfaceof the receptacle, unless willfully done with additional tooling. Inaddition the grounding prong member is spring-biased so that it mayengage grounding means placed on a receptacle for grounding safety whena receptacle does not have provision for receiving a grounding prongwithin an aperture.

This invention is not to be limited by the embodiment shown in thedrawings and described in the description, which are given by way ofexample and not of limitation, but only in accordance with the scope ofthe appended claims.

I claim:

1. An electrical connector plug which comprises:

a. a body member, said body member having a longitudinal axis;

b. a plurality of electrically conductive contact prong members, saidelectrical contact prong members being mounted to said body member inposition to protrude from said body member parallel to said longitudinalaxis, said electrical contact prong members being adapted to be attachedto electrical conductors for transmission of electrical current;

c. at least one grounding prong member, said grounding prong membercomprising an axially shiftable conductive element being carried by thebody member, and shiftable within the body member, said grounding prongmember being adapted to protrude axially from and to be retracted intosaid body member, said grounding prong member being adapted to beattached to a grounding conductor;

d. latch means movably mounted to the plug selectively to permit or tolimit axial movement of the axially shiftable conductive element;

e. bias spring means, said bias spring means biasing the axiallyshiftable conductive element toward its protruding position;

f. sensing finger means movably mounted to the plug and so disposed andarranged relative to the latch means as to move the latch means topermit axial movement of the axially shiftable conductive element intothe body member when said sensing finger undergoes an axial force, saidsensing finger means projecting beyond the end of said axially shiftableconductive element unless it is retracted by said axial force.

2. A device according to claim 1 in which said sensing finger means isdisposed adjacent to said grounding prong member.

3. A device according to claim 1 in which said grounding prong memberincludes a conductive outer tubular member fixed to said body member,said axially shiftable conductive element being slidably mounted withinsaid outer tubular member, said axially shiftable conductive elementbeing adapted to protrude from said body member parallel to saidlongitudinal axis, the outer tubular member being adapted to be attachedto a grounding conductor.

4. A device according to claim 3 in which the axially shiftable tubularelement includes a bore through which the sensing finger means passes,and in which it is axially shiftable, the biasing on the sensing fingermeans biasing one of its ends to protrude beyond the respective end ofthe axially shiftable conductive element.

5. A device according to claim 4 in which the latch means is pivotallymounted to the axially shiftable conductive element, in which the outertubular member includes an axially extending slot, and in which thelatch means includes an upstanding arm adapted to enter said last-namedslot and engage said wall of said axial passage to prevent retraction ofthe axially shiftable conductive element except when the sensing fingermeans is retracted into the axially shiftable conductive element.

6. A device according to claim 5 in which the bias spring means alsobiases the latch means toward its inter-engaged position.

7. A device according to claim 5 in which the said last-named slot is ofsuch length as to permit a limited, but not total, amount of retractionof the axially shiftable conductive element even when the sensing fingermeans is not retracted. Y

8. A device according to claim 3 in which said latch means is formed asa bell-crank lever means adapted to lock said axially shiftableconductive element in a protruding axial position with respect to saidbody memher.

9. A device according to claim 3 in which cap plug means of electricallyconductive material is fixedly inserted in said axially shiftableconductive element, said cap plug means being adapted to enter anelectrical receptacle as a leading part of said grounding prong member.

10. A device according to claim 9 in which said cap plug means isconstructed of a magnetically permeable material having electricallyconductive properties.

11. A device according to claim 9 in which said cap plug means isconstructed of a magnetized material having eletrically conductiveproperties.

12. A device according to claim 9 in which said cap plug means has anaxial bore therethrough, said bore being adapted to slidably mount saidsensing finger means, said sensing finger means being adapted toprotrude from said cap plug means.

13. A device according to claim 8 in which said outer tubular member andsaid axially shiftable conductive element have co-registering axiallyextending slots, and the bell-crank lever means includes an upstandingarm, said slot in said outer tubular member being aligned with said slotin said axially shiftable conductive element, said slots being adaptedto engage the upstanding lever arm of said bell-crank lever means forlocking said axially shiftable conductive element in a protrudingposition with respect to said body member, said spring bias means beingadapted to prevent said sensing finger means from releasing saidbell-crank lever means when said sensing finger means is contacted by afleshlike resilient material whereby said fleshlike resilient materialwill yield until contact is made with said grounding prong to push backsaid axially shiftable conductive element against said spring bias meansthereby to lock said upstanding arm of said bell-crank lever means insaid slot of'said outer tubular member to prevent sliding of saidaxially shiftable conductive element into said body member.

14. In combination: an electrical receptacle having sockets forreceiving electrical conductor prongs, an electrical plug, andinterconnecting grounding means, said combination comprising:

grounding plate means mounted to the electrical receptacle and having aconductive contact surface facing away from the sockets; V

a plug body member, said plug body member having a longitudinal axis;

a plurality of electrically conductive contact prong members, saidelectrical contact prong members being mounted to said body member andprotruding from said body member parallel to said longitudinal axis,said electrical contact prong members being adapted to be attached toelectrical conductors for transmission of electrical current;

at least one grounding prong member, said grounding prong membercomprising an axially shiftable conductive element being carried by thebody member, and shiftable in and embraced by the wall of an axialpassage in the body member, said grounding prong member being adapted toprotrude axially from and to be retracted into said body member, saidgrounding prong member being adapted to be attached to a groundingconductor;

latch means movably mounted to the plug selectively to permit or tolimit axial movement of the axially shiftable conductive element;

bias spring means, said bias spring means biasing the axially shiftableconductive element toward its protruding position;

sensing finger means movably mounted to the plug and so disposed andarranged relative to the latch means as to move the latch means topermit axial movement of the axially shiftable conductive element intothe body member when said sensing finger undergoes an axial force, saidsensing finger means projecting beyond the end of said axially shiftableconductive element unless it is retracted by said axial force.

15. A combination according to claim 14 in which said sensing fingermeans is disposed adjacent to said grounding prong member.

16. A combination according to claim 14 in which said grounding prongmember includes a conductive outertubular member fixed to said bodymember, said axially shiftable conductive element being slidably mountedwithin said outer tubular member, said axially shiftable conductiveelement being adapted to protrude from said body member parallel to saidlongitudinal axis, said grounding prong member being adapted to beattached to a grounding conductor.

17. A combination according to claim 16 in which the axially shiftabletubular element includes a bore through which the sensing finger meanspasses, and in which it is axially shiftable, the biasing on the sensingfinger means biasing one of its ends to protrude beyond the respectiveend of the axially shiftable conductive element.

18. A combination according to claim 17 in which the latch means ispivotally mounted to the axially shiftable conductive element, in whichthe outer tubular member includes an axially extending slot, and inwhich the latch means includes an upstanding arm adapted to enter saidlast-named slot and engage said wall of said axial passage to preventretraction of the axially shiftable conductive element except when thesensing finger means is retracted into the axially shiftable conductiveelement.

19. A combination according to claim 18 in which the bias spring meansalso biases the latch means toward its inter-engaged position.

20. A combination according to claim 18 in which the said last-namedslot is of such length as to permit a limited, but not total, amount ofretraction of the axially shiftable conductive element even when thesensing finger means is not retracted.

21. A combination according to claim 16 in which said latch means isformed as a bell-crank lever means adapted to lock said axiallyshiftable conductive element in a protruding axial position with respectto said body member.

22. A combination according to claim 16 in which cap plug means ofelectrically conductive material is fixedly inserted in said axiallyshiftable conductive element, said cap plug means being adapted to enteran electrical receptacle as a leading part of said grounding prongmember.

23. A combination according to claim 22 in which said cap plug means isconstructed of a magnetically permeable material having electricallyconductive properties.

24. A combination according to claim 22 in which said cap plug means isconstructed of a magnetized material having electrically conductiveproperties.

25. A combination according to claim 22 in which cap plug means has anaxial bore therethrough, said bore being adapted to slidably mount saidsensing finger means, said sensing finger means being adapted toprotrude from said cap plug means.

26. A combination according to claim 21 in which said outer tubularmember and said axially shiftable conductive element have co-registeringaxially extending slots, and the bell-crank lever means includes anupstanding area, said slot in said outer tubular member being alignedwith said slot in said axially shiftable conductive element, said slotsbeing adapted to engage the upstanding lever arm of said bell-cranklever means for locking said axially shiftable conductive element in aprotruding position with respect to said body member, said spring biasmeans being adapted to prevent said sensing finger means from releasingsaid bell-crank lever means when said sensing finger means is contactedby a fleshlike resilient material whereby said fleshlike resilientmaterial will yield until contact is made with said grounding prong topush back said axially shiftable conductive element against said springbias means thereby to lock said upstanding arm of said bell-crank levermeans in said slot of said outer tubular member to prevent sliding ofsaid axially shiftable conductive element into said body member.

27. A combination according to claim 14 in which said grounding platemeans is constructed of magnetically permeable electrically conductivematerial.

28. A combination according to claim 14 in which said grounding platemeans is constructed of magnetized electrically conductive material.

1. An electrical connector plug which comprises: a. a body member, saidbody member having a longitudinal axis; b. a plurality of electricallyconductive contact prong members, said electrical contact prong membersbeing mounted to said body member in position to protrude from said bodymember parallel to said longitudinal axis, said electrical contact prongmembers being adapted to be attached to electrical conductors fortransmission of electrical current; c. at least one grounding prongmember, said grounding prong member comprising an axially shiftableconductive element being carried by the body member, and shiftablewithin the body member, said grounding prong member being adapted toprotrude axially from and to be retracted into said body member, saidgrounding prong member being adapted to be attached to a groundingconductor; d. latch means movably mounted to the plug selectively topermit or to limit axial movement of the axially shiftable conductiveelement; e. bias spring means, said bias spring means biasing theaxially shiftable conductive element toWard its protruding position; f.sensing finger means movably mounted to the plug and so disposed andarranged relative to the latch means as to move the latch means topermit axial movement of the axially shiftable conductive element intothe body member when said sensing finger undergoes an axial force, saidsensing finger means projecting beyond the end of said axially shiftableconductive element unless it is retracted by said axial force.
 2. Adevice according to claim 1 in which said sensing finger means isdisposed adjacent to said grounding prong member.
 3. A device accordingto claim 1 in which said grounding prong member includes a conductiveouter tubular member fixed to said body member, said axially shiftableconductive element being slidably mounted within said outer tubularmember, said axially shiftable conductive element being adapted toprotrude from said body member parallel to said longitudinal axis, theouter tubular member being adapted to be attached to a groundingconductor.
 4. A device according to claim 3 in which the axiallyshiftable tubular element includes a bore through which the sensingfinger means passes, and in which it is axially shiftable, the biasingon the sensing finger means biasing one of its ends to protrude beyondthe respective end of the axially shiftable conductive element.
 5. Adevice according to claim 4 in which the latch means is pivotallymounted to the axially shiftable conductive element, in which the outertubular member includes an axially extending slot, and in which thelatch means includes an upstanding arm adapted to enter said last-namedslot and engage said wall of said axial passage to prevent retraction ofthe axially shiftable conductive element except when the sensing fingermeans is retracted into the axially shiftable conductive element.
 6. Adevice according to claim 5 in which the bias spring means also biasesthe latch means toward its inter-engaged position.
 7. A device accordingto claim 5 in which the said last-named slot is of such length as topermit a limited, but not total, amount of retraction of the axiallyshiftable conductive element even when the sensing finger means is notretracted.
 8. A device according to claim 3 in which said latch means isformed as a bell-crank lever means adapted to lock said axiallyshiftable conductive element in a protruding axial position with respectto said body member.
 9. A device according to claim 3 in which cap plugmeans of electrically conductive material is fixedly inserted in saidaxially shiftable conductive element, said cap plug means being adaptedto enter an electrical receptacle as a leading part of said groundingprong member.
 10. A device according to claim 9 in which said cap plugmeans is constructed of a magnetically permeable material havingelectrically conductive properties.
 11. A device according to claim 9 inwhich said cap plug means is constructed of a magnetized material havingeletrically conductive properties.
 12. A device according to claim 9 inwhich said cap plug means has an axial bore therethrough, said borebeing adapted to slidably mount said sensing finger means, said sensingfinger means being adapted to protrude from said cap plug means.
 13. Adevice according to claim 8 in which said outer tubular member and saidaxially shiftable conductive element have co-registering axiallyextending slots, and the bell-crank lever means includes an upstandingarm, said slot in said outer tubular member being aligned with said slotin said axially shiftable conductive element, said slots being adaptedto engage the upstanding lever arm of said bell-crank lever means forlocking said axially shiftable conductive element in a protrudingposition with respect to said body member, said spring bias means beingadapted to prevent said sensing finger means from releasing saidbell-crank lever means when said sensing finger means is contacted by afleshlike resilient material whereby said fleshlike resilient materialwill yield until contact is made with said grounding prong to push backsaid axially shiftable conductive element against said spring bias meansthereby to lock said upstanding arm of said bell-crank lever means insaid slot of said outer tubular member to prevent sliding of saidaxially shiftable conductive element into said body member.
 14. Incombination: an electrical receptacle having sockets for receivingelectrical conductor prongs, an electrical plug, and interconnectinggrounding means, said combination comprising: grounding plate meansmounted to the electrical receptacle and having a conductive contactsurface facing away from the sockets; a plug body member, said plug bodymember having a longitudinal axis; a plurality of electricallyconductive contact prong members, said electrical contact prong membersbeing mounted to said body member and protruding from said body memberparallel to said longitudinal axis, said electrical contact prongmembers being adapted to be attached to electrical conductors fortransmission of electrical current; at least one grounding prong member,said grounding prong member comprising an axially shiftable conductiveelement being carried by the body member, and shiftable in and embracedby the wall of an axial passage in the body member, said grounding prongmember being adapted to protrude axially from and to be retracted intosaid body member, said grounding prong member being adapted to beattached to a grounding conductor; latch means movably mounted to theplug selectively to permit or to limit axial movement of the axiallyshiftable conductive element; bias spring means, said bias spring meansbiasing the axially shiftable conductive element toward its protrudingposition; sensing finger means movably mounted to the plug and sodisposed and arranged relative to the latch means as to move the latchmeans to permit axial movement of the axially shiftable conductiveelement into the body member when said sensing finger undergoes an axialforce, said sensing finger means projecting beyond the end of saidaxially shiftable conductive element unless it is retracted by saidaxial force.
 15. A combination according to claim 14 in which saidsensing finger means is disposed adjacent to said grounding prongmember.
 16. A combination according to claim 14 in which said groundingprong member includes a conductive outer tubular member fixed to saidbody member, said axially shiftable conductive element being slidablymounted within said outer tubular member, said axially shiftableconductive element being adapted to protrude from said body memberparallel to said longitudinal axis, said grounding prong member beingadapted to be attached to a grounding conductor.
 17. A combinationaccording to claim 16 in which the axially shiftable tubular elementincludes a bore through which the sensing finger means passes, and inwhich it is axially shiftable, the biasing on the sensing finger meansbiasing one of its ends to protrude beyond the respective end of theaxially shiftable conductive element.
 18. A combination according toclaim 17 in which the latch means is pivotally mounted to the axiallyshiftable conductive element, in which the outer tubular member includesan axially extending slot, and in which the latch means includes anupstanding arm adapted to enter said last-named slot and engage saidwall of said axial passage to prevent retraction of the axiallyshiftable conductive element except when the sensing finger means isretracted into the axially shiftable conductive element.
 19. Acombination according to claim 18 in which the bias spring means alsobiases the latch means toward its inter-engaged position.
 20. Acombination according to claim 18 in which the said last-named slot isof such length as to permit a limited, but not total, amount ofretraction of the axially shiftable conductive element even when thesensing finger means is not retracted.
 21. A combination according toclaim 16 in which said latch means is formed as a bell-crank lever meansadapted to lock said axially shiftable conductive element in aprotruding axial position with respect to said body member.
 22. Acombination according to claim 16 in which cap plug means ofelectrically conductive material is fixedly inserted in said axiallyshiftable conductive element, said cap plug means being adapted to enteran electrical receptacle as a leading part of said grounding prongmember.
 23. A combination according to claim 22 in which said cap plugmeans is constructed of a magnetically permeable material havingelectrically conductive properties.
 24. A combination according to claim22 in which said cap plug means is constructed of a magnetized materialhaving electrically conductive properties. said
 25. A combinationaccording to claim 22 in which cap plug means has an axial boretherethrough, said bore being adapted to slidably mount said sensingfinger means, said sensing finger means being adapted to protrude fromsaid cap plug means.
 26. A combination according to claim 21 in whichsaid outer tubular member and said axially shiftable conductive elementhave co-registering axially extending slots, and the bell-crank levermeans includes an upstanding area, said slot in said outer tubularmember being aligned with said slot in said axially shiftable conductiveelement, said slots being adapted to engage the upstanding lever arm ofsaid bell-crank lever means for locking said axially shiftableconductive element in a protruding position with respect to said bodymember, said spring bias means being adapted to prevent said sensingfinger means from releasing said bell-crank lever means when saidsensing finger means is contacted by a fleshlike resilient materialwhereby said fleshlike resilient material will yield until contact ismade with said grounding prong to push back said axially shiftableconductive element against said spring bias means thereby to lock saidupstanding arm of said bell-crank lever means in said slot of said outertubular member to prevent sliding of said axially shiftable conductiveelement into said body member.
 27. A combination according to claim 14in which said grounding plate means is constructed of magneticallypermeable electrically conductive material.
 28. A combination accordingto claim 14 in which said grounding plate means is constructed ofmagnetized electrically conductive material.